In local-probe data storage devices, stored data is represented on a storage surface by the presence and absence of indentations, or “pits”, which are formed in the storage surface by a probe of the device. One example of such a device, based on the AFM (Atomic Force Microscope), is detailed in IBM Journal of Research & Development, Volume 44, No. 3, May 2000, pp 323–340, “The ‘Millipede’-More Than One Thousand Tips for Future AFM Data Storage”, Vettiger et al., and the references cited therein. This device employs an integrated array of individually-addressable probes, where each probe here is a nanometer-sharp tip mounted on the end of a microfabricated cantilever. The array of probes can be moved over the surface of a storage medium in the form of a polymer substrate, such that each probe can read and write data within its own storage field on the surface. In particular, a resistive heater element is formed in the cantilever body to enable heating of the probe tip to a write temperature on application of a drive signal in the write-scan mode. This causes local heating of the polymer surface at the point of contact with the tip, allowing the tip to penetrate the surface to create a pit. Such a pit typically represents a bit of value “1”, a bit of value “0” being represented by the absence of a pit at a bit-position on the storage surface. In a read-scan mode, the thermomechanical probe mechanism can be used to read data by sensing the deflection of the cantilever as the tip is moved over the pattern of pits and “no pits” at bit-positions on the storage surface. In this particular device, the heater also acts as a read sensor by virtue of its temperature-dependent resistance. During read-scanning, a drive signal is applied to heat the heater to a lower temperature than in the write mode so that the tip does not get hot enough to deform the storage surface. However, as the tip scans the storage surface, heat transfer between the heater and storage surface is more efficient when the tip enters a pit than when there is no pit at a bit-position. The temperature, and hence resistance, of the heater will therefore be less when reading a “1” than a “0”. A read signal dependent on the resistance of the heater during read scanning can thus be processed by detection circuitry to detect the data at the scanned bit-positions.
One of the most critical issues for read detection in local-probe storage devices is the high resolution required to extract the signal containing the information on whether a read bit is a “1” or a “0”. In the device described above, for example, reading a “1” typically produces a relative change in the sensor resistance of the order of ΔR/R=10−4. The useful information signal can thus be viewed as a small signal superimposed on a very large offset signal. The presence of various other offsets, such as those resulting from manufacturing tolerances, makes the task of read detection even more difficult. For example, the use of simple threshold detection is not feasible in the presence of such offset signals due to the high precision required for analog-to-digital conversion.
Our international patent application published under WO 03/021592 A2 discloses read detection apparatus for local-probe storage devices where the detection apparatus incorporates an offset compensation mechanism. In the system described, a dedicated reference cantilever probe is provided in the array. During read-scanning, the reference probe reads a dedicated reference area of the storage surface in which all bits are equal to “0”. The sensor read signal from the reference probe serves as a reference signal. This reference signal is subtracted from the read signal obtained from each probe reading a data field in operation, thereby reducing the dynamic range of the read signal. The resulting difference signal is low-pass filtered to limit the bandwidth of high-frequency noise, and a high-pass filter then eliminates residual offsets and low-frequency noise. The high-pass filter samples the difference signal at a rate of 1/T, and subtracts each sample from the succeeding sample to generate a ternary difference signal. The delay factor T here corresponds to the read interval, i.e. the time between reading of successive bits on the storage surface. The ternary difference signal is supplied to an analog-to-digital converter in the form of a three-level decision element which determines whether the read bit is a “0” or a “1”.
However, note that the signal generated by the reference probe is a noisy signal. During the read process, a cantilever in the foregoing system can be modelled as a variable resistance at a temperature of about 350° C., so that thermal noise (Johnson's noise) must be taken into account, as well as electronics noise and media noise that will be present in the reference signal. The level of these disturbances is similar to the level of the noise in the desired read signal, and hence use of a reference probe for offset compensation leads to a loss of about 3 dB in signal-to-noise ratio. Secondly, in an array of probes the read signal samples are typically obtained by applying a drive pulse to the probes in a column (or a row) of the array, low-pass filtering the individual read signals, and finally sampling the filter output signals. This process is repeated sequentially until all columns (or rows) of the array have been addressed. The array is then moved to the next bit-position and the read process restarted from the first column (or row). The time between two drive pulses corresponds to the time it takes for a probe to move from one bit-position to the next. As constant velocity is usually assumed, the pulses are periodically applied to each probe. If only one probe is employed as a reference probe, the rate of pulses applied to the reference probe will be different from the rate of pulses applied toga data probe. If the pulse rate is high in particular, the period between pulses applied to the reference probe may be of the order of the probe thermal time constant. As a consequence, the response of the reference probe will not match the responses of the data probes, and the accuracy of offset compensation will be degraded. To alleviate this problem, a reference probe per column (or row) might be used, each reading a corresponding reference field. However, this approach increases the implementation complexity and reduces efficiency of the system.